This invention relates to regulator circuits for television receivers.
In many SCR horizontal deflection circuits, energy is coupled to the deflection circuit from a source of B+ operating voltage through an input choke that is coupled to the commutating switch of the deflection circuit. Conventional regulators for these circuits have included saturable reactors, the inductance of which is controlled to achieve regulation, or have included various types of switching arrangements.
One type of prior art regulator provides for forward regulation of the input operating current. In these forward regulators, an SCR is coupled in series with the B+ supply and the input choke. A phase controlled oscillator responsive to the energy level within the deflection circuit gates the SCR into conduction during the commutating interval of each deflection cycle. The SCR is commutated off during the noncommutating interval as the voltage across the commutating switch causes the current through the input choke and the SCR to decrease below the SCR holding current level. Regulation is achieved by varying the turn-on time of the SCR, thereby controlling the amount of energy provided by the B+ supply to the deflection circuit.
Since the regulator SCR is commutated off by the commutating voltage, further circuitry functioning as a solid state circuit breaker is needed to provide short circuit protection should the commutating voltage be insufficient to turn off the SCR or should it be absent completely due to a shorted commutating switch. Prior art circuitry has included another SCR device in series between the output of the AC line rectifier circuit and the B+ filter capacitor. If the commutating voltage disappeared or too much operating current is drawn, gating signals are removed from the circuit breaker SCR, thereby open-circuiting the power supply circuit. Such protection circuitry requires two power devices capable of operating with relatively large currents flowing through them and relatively large voltages impressed across the devices. It is desirable to develop circuitry which will eliminate the need for two relatively large and expensive SCR's by combining both regulating and circuit breaker functions in a circuit requiring only a single power device.
Other prior art circuitry combines the function of forward regulation and circuit breaker protection into a single transistor device in series with the unregulated B+ supply and the input choke inductor of the SCR horizontal deflection circuit. A modulated signal coupled to the transistor base turns on the transistor during the commutating interval and then turns off the transistor during the noncommutating interval, thereby providing regulation. Should faulty operation be encountered, the base signals are removed, thereby providing circuit breaker protection.
By turning off the transistor under normal operation instead of commutating it off, that is, instead of using the commutating voltage to decrease the current in the collector-emitter transistor path to zero and reverse bias that junction, relatively large currents must flow under normal conditions when the transistor is cut off, requiring a transistor capable of withstanding this turn-off stress. Furthermore, the collector current must now be transferred to a damping snubber network undesirably dissipating the collector current even under normal conditions.